The present invention relates to a fin, and more particularly to a fin with movable flap. The flap is pushed by water to open a hole on a blade of the fin when a diver sweeps a leg upward, allowing the diver to sweep the leg upward with reduced effort. And the flap is pushed by water to close the hole on the blade when the diver""s leg is swept downward, allowing the diver to kick water more powerfully and advance under water more quickly.
It is known that a diver would alternately sweep two legs upward and downward like swimming in free stroke in order to advance under water. However, as a matter of fact, it is only the downward swept leg that kicks water to cause a flat blade of a fin worn on that leg to resist water and therefore propels the diver forward. The other leg is upward swept only to prepare for a next kick of water.
A conventional fin typically includes a solid and flat blade. When the diver sweeps one leg upward, the blade of the fin worn on that leg encounters a downward water resistance that does not help but rather hinder the diver in his or her forward movement. Under this condition, it would be difficult for the diver to alternately move two legs at large sweep angle to advance under water quickly. Moreover, human""s leg is so structured that it tends to more easily kick forward than extend backward and upward. The downward water resistance against the blade of the fin on the diver""s upward swept leg would therefore easily harm the diver by causing a cramped leg and largely consumed body strength.
Furthermore, in diving from a boat or a bank, there is a considerably big force generated at the instant the flat blade contacts with water. This strong force acting on the flat blade of the fin is borne by the diver""s ankle. In the event the diver enters the water at an unfavorable angle, the ankles would be easily sprained and the diver tends to lose balance and collide his head against the boat or bank to cause serious result.
Therefore, it is desirable to develop an improved fin to eliminate the above-mentioned problems in diving.
A primary object of the present invention is to provide a fin that enables a diver to advance under water with reduced effort.
To achieve this and other objects, the fin according to the present invention mainly includes a blade provided with one or more holes and one or more one-way movable flaps connected to the blade to cover the hole or holes on the blade. The flap each has a moving edge opposite to the fixing edge to movably overlap the blade. When a diver alternately sweep two legs up and down under water, the movable flap on a top surface of the fin worn on the upward swept leg is pushed downward by the resistance from the water to open the hole. The same resistance vanishes when the hole is opened, allowing the diver to sweep the leg upward with reduced effort. On the other hand, the flap on the fin worn on the downward swept leg is pushed upward by the resistance from the water to close the hole, allowing the diver to kick water more powerfully and advance more quickly.